[sdiy] ASM1 - MAT & caps

[Ian Fritz]

There has been some discussion of this point here before, and people never 
really came to any agreement. A Vbe mismatch could be caused by material 
variation and/or defects, in which case the measured dVbe could certainly 
correlate with a mismatch in the Is temperature variation. However, it is 
also possibe to have a mismatch in Vbe due to purely geometric effects 
(different meas areas) without a temperature-dependent mismatch in Is. In 
this case the Vbe mismatch produces an offset, but no temperature 
problems.  So it's a question of which factor in Is is causing the mismatch 
in Vbe.

   Ian



At 07:12 PM 2/6/2002, Scott Bernardi wrote:
>There's actually two temperature dependencies in the Ic vs Vbe relationship:
>Ic = Is*exp(-q/kT)
>The Is term has a temperature dependence that approximately doubles every 10
>degrees C. When you use two matched transistors in an exponentially converter,
>they cancel each other out. What's left over is the -q/kT term that we use
>tempco resistors to compensate for.
>How well the Is terms cancel depends on the matching of the transistors and
>the thermal coupling.  A delta Vbe is an indication of mismatch. 150uV is
>plenty good, however.